Automatic frequency control system



April 12, 1949.. c. TRAVIS 2,467,345

AUTOMATIC FREQUENCY CONTROL SYSTEM ATTORN EY oc/1A dica um? April 12, 1949. C'. TRAVIS AUTOMATIC FREQUENCY CONTROL SYSTEM Original Filed May 3; 1935 3 Sheets-Sheet 2 Tagli- /f 50 i r1 C @naif Ticil I BY 'A ATTORNEY April 12, 1949. Q TRAVIS 2,467,345

AUTOMATIC FREQUENCY CONTROL SYSTEM u I original Filed May s, 1935 s sheets-sheet 3 BY )195. MM

ATTO R N EY Patented Apr. 12, 1949 UNITED STATES PATENT OFFICE AUTOMATIC FREQUENCY CONTROL SYSTEM Original application May 3, 1935, Serial No.

19,563, now Patent No. 2,357,984, dated September 12, 1944.

Divided and this application March 27, 1944, Serial No. 528,317

(Cl. Z50-36) 1 Claim. 1

The presen-t invention relates to frequency control systems, and more particularly to improvements in automatic frequency control networks. This application is a division of a-pplication Serial No. 19,563 filed May 3, 1935, patented September l2, 1944, :as U. S. Patent No. 2,357,984.

There has been disclosed in U. S. Patent No. 2,294,100, patented August 25, 1942, various circuits for automatically varying the tuning of a resonant circuit independently of the normal tuning device. In these previ-ous systems rthe results sought to be accomplished have in general been attained. However, additional investigation and experimentation has resulted in the development of further improvements in automatic frequency control systems.

The automatic frequency control systems to be explained in rdetail in a later portion of this specification involve in general two distinct units employed in a superheterodyne receiver. One of :these units is a frequency discriminator, or frequency sensitive detector, that generates a bias varying with changes of the intermediate frequency signal carrier. The other unit is a control thatv is acted upon by the aforementioned bias, and whose function it is to vary the local oscillator frequency in a desired sense. The two units are so coordinated that if the intermediate frequency carrier tends to shift from its midband frequency value, the oscillator frequency changes sufliciently to restore proper alignment.

The automatic frequency control systems which comprise the subject matter of the present application are not only useful in :facilitating the manual tuning operation of a superheterodyne receiver, but are also highly useful in maintaining the local oscillator frequency at a desired carrier setting f-or a long -period of time, and throughout operation of the reeciver during this period. f In addition to its aid and benefit in connection with the manual tuning of superheterodyne re-I ceivers, the presently disclosed frequency control systems can be used with benefit in connection with the automatic tuning of radio receivers of the superheterodyne type. In such instances the frequency control network functions as a Vernier device to accurately tune the oscillator circuit after the automatic tuning mechanism has substantially .adjusted the tuning device to its desired station position.

Accordingly, it may be stated that it is one of the primary objects of the present invention to provide improved automatic frequency control networks which are not only capable of usage in such a manner as to secure the aforementioned results, but are Ialso constructed so as to function in a positive and reliable manner.

Another important object of the Ipresent invention `to provide :automatic frequency control networks based on an operating principle which is substantially different from that utllized in connection with the circuits disclosed in the aforementioned patent. The presentl operation is based upon the fact that when the plate of the frequency control tube is coupled into the high potential side of .the resonant circuit to be adjusted in tuning, and the grid of the control tube is excited by a voltage out of phase with that appearing across the resonant circuit, then the plate current in the control tube is similarly out of phase with the resonant circuit voltage, and :the control tube presents a reactive effect across the resonant circuit; sthe magnitude of this reactive effect varies according to the frequency control bias impressed upon the control grid of the control tube by the frequency discriminator network.

Another object of the present invention is to provide an automatic frequency control system for a superheterodyne receiver, the Vsystem comprising a frequency discriminator network functioning to -provide a direct current vbias which varies in magnitude with the shifting frequency of the intermediate frequency carrier; a reactance control tube being used in operative association with the local oscillator tank circuit in such a mann-er that the plate current of the control tube is out of phase with the tank circuit voltage whereby the control tube simulates a predetermined reaotance across the tank circuit, the magnitude of the control tube reactance varying Iaccording to the discriminator bias which is impressed upon the control grid of the control tub-e.

Another object of the present invention may be said :to reside in the Aprovision of a tunable oscillation circuit which is capable of being 'automatically etuned to different frequencies in a predetermined frequency range, there being an automatic frequency control system operatively associ-ated with the Aoscillation circuit in such a manner that the oscillation circuit is accurately `and automatically tuned to resonance with a desired signal carrier when the tuning means of the oscillation circuit is adjusted to a predetermined frequency, there being provided additional means for rendering the frequency con-trol operative as soon as the oscillation circuit has been tuned to approximate resonance with said predetermined frequency.

Still other objects of the invention are to imd prove generally the simplicity and eiciency of automatic frequency control systems lfor radio receivers, :and more especially to .provide such systems in an economical and practical manner; the control systems of the present invention being particularly characterized .by their reliability and accuracy in operation.

Still other features will best -be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically'several circuit organizations whereby the invention may be carried into effect.

In the drawings:

Fig 1 diagrammatically shows a superheterodyne receiving system embodying onel form of the present invention,

Fig. 2 shows a modification of the frequency control network in Fig. 1,'

Fig.. 3.,shows a. further modification of the frequency control 4network in Fig, 1',

Figs. 4,. andV show additionalmodications of the frequency controllnetworkof Fig. 1.

Referring now to the accompanying drawings, whereinlikereference characters in the different figures designate similar circuit elements, the receiving system shown in Fig. 1 isof. the superheterodyne type,.and is shown as embodying an automatic frequency control network whose general organization is similar to that disclosed and claimed inthe aforesaidPatent 2,294,100.

The receivingv system embodies generally a grounded antenna circuit A which feeds a radio frequency amplifier I having a tunable input which isresonated to the desired signal frequency.. The output. energy, of the amplifier I is impressed; upon the tunable input circuit 3 of the first detector tube 2, the latter being shown by` way of illustration as of the 6A7 type. The plate circuit ofthe tube.2.incl.udes a tuned circuit 4" which is resonated to theoperating intermediate frequency of. theY system,.and the latter may be chosen to havev a value, for example, of the. order of 460 kilocycles (kc). The intermediate frequency energy producedin the circuit 4# may then bev amplified in one or more stages of intermediatefrequency amplification, and by way oi' exampletwo sucncascaded amplifier stages 5 and 6 are. shown. The numeral] denotes the tuned input circuit ofthe amplier 5, Whereas the numeraly B denotes the tuned output. circuit ofthe amplifier.

The intermediate frequency` amplifier 6 is provided with ,a tuned input circuit 9 and a tuned output circuit IIJ; it being. pointed out that the circuits 4, 1,' 8, 9 and I0, as well as the input circuit I'I to the'second detector, are each resonatedto the operating intermediate frequency of the system. The. second detectory or demodulator of the system may be of any well known type, and may follow the construction, for example, shown in the. aforesaid U. S. Patent 2,294,100. The local oscillator network comprises a tube I2 which is provided with a tunable oscillator tankv circuit. The tanky circuit includes the tuning condenser I3`, and the series-arranged coil I4 and resistance Rare connectedin shunt with the condenser I3'. The high potentialr side of the tank circuitis connectedto the control grid of tube I2 through a blocking condenser I5, the usual' grid return resistor ISbeing connected to ground from thegridside of condenser I5.

The cathode of' oscillator tube I2 in grounded, and the plate thereof is regenerativel'y. coupled to the tank circuit by means of'a coil I4 which d is magnetically coupled to coil Ill The positive potential required for the plate of oscillator tube If. is fed to the plate through" the feedback coil ifi', and the' low alternating potential side of the tank circuit is connected to a source of positive potential. This latter positive potential is provided for the plate of the frequency control tube toA be described in further detail at a later point. The locally produced oscillations are impressed upon the grid I'I, which is the nearest to the cathode of tube 2. The grid I'I is connected to the high alternating potential side of the oscillator tank circuit through a blocking condenser I 8, the grid side of condenser I8 being connected to the cathode side of the grid bias network 20 by a resistor I9.v

t will be observed'that a dotted line denotes the mechanical uni-control device usually employed for operating the rotors of the Variable tuning condensers of the signal and local oscillator circuits. It is to be clearly understood that the dottedline signifies the mechanical coupling be'- tweenthe rotors of'condensers I3 and 3f, and also' with the rotors of thev variable condenser usually employed in the input of amplifier I. The fre'- quency changing function in tube 2 is accom'- plished by means of electroniccoupling, and'v this action is so well known to those skilled in the art' at thepresent time thata mere reference thereto is believed suilicient The signal carrier amplitude at the demod'- ulator input circuit II is maintained substantially constantover a wide'range of carrier am'- plitude Variationy at the'. signal collector A by means of an automatic volume'contr'ol'arrangement, the latter being denoted as AVC in Fig. 1. The varying negative bias for. securing the automatic amplification regulation is derived fromv the diode network operatively associated with the driver tube 2'I The latter tube maybev of the`78 or, 6D6 type, and has its control grid. coupled', for the impression thereon of intermediate frequency voltage to the high alternatingpotential side of the tuned output' circuit 8' of amplifier 5. The' signal'path connected: toV the control grid of' tube 2|' includes a direct current' blockingcondenser 22', the controlv grid'being connected to ground through a gridreturn resistor 23.

The plate of tube ZI is connected to a sourceV of positive potential through two paths; one of these paths includes' the coil P1, while the other path includes the coil'Pz. Each of coils P1 andPz is shunted bya condenser 2L The AVC diodel networkcomprises theauxiliaryanode 25 which is disposed adjacent a portion of the cathode of tube 2| outside the main electron streamv to the plate ofjthe tube. Thegrounded-'cathode lead of tube' 2 IV includes the control gridbias network 26; the resistor 23 actingv as-a conducting path for the gridbias ofthe control grid of the driver tube. Intermediate'frequency energy is impressed upon the diode anode2'5'through the directeurrent blocking condenserZ'I whichis connected4 between'the plate of'tube 2IV andthe diodel anode 25; The diode anode 25-has connected in circuit therewith a loadtresistor'28', the latter developing' a direct current voltage across it when the inter'- mediate frequencyv carrier amplitude attains a magnitudeabove a predetermined intensity level.

The anode side of resistor 28 is connected to the gain control electrodes ofthe controlled sig,- naltransmission tubes. In the present case the signal grids'may act, as thev gain control elec-A trodes, and the AVC lead; is to be understood as the' controlled transmission tubes. The AVC lead -isfconnected to the anode side of resistor 28 through a resistor 29 which acts to suppress pulsating components of rectified intermediate frequency voltage; the condenser 30 connected to ground cooperating with resistor 29 to provide a suitable time constant network for the AVC arrangement.

termediate frequency carrier amplitude at circuit 8 increases, the negative bias applied to the control grids of the regulated tubes by the AVC nework increases, thereby reducing the amplification of each tube, and in this way the signal amplitude at the demodulator input circuit is maintained substantially uniform regardless of the signal amplitude variation at collector A. The frequency discriminator network comprises the double diode tube 3|, and the latter may be,

by way of specific illustration, of the 85 type with the triode section thereof unused. The anodes of tube 3|, these anodes being designated by numerals 32 and 33, are connected to opposite sides' of a resistor which develops the automatic frequency control (AFC) bias. This resistor has the cathode of tube 3| connected to the mid-point thereof, and therefore a single resistor may be employed or a pair of resistors of equal magnitude may be used instead. In any event the numeral 34 designates the resistor section between the cathode of tube 3| and the anode 32.

The resistor section 34 designates the load between the cathode of tube 3| and the diode anode 33. Each of the resistor sections 34 and 34 is shunted vby a bypass condenser, and the anode side of resistor 34 is grounded. The secondary coil S1, which is magnetically coupled to primary P1, is connected between anode 32 and resistor section 34. The secondary coil S2 is magnetically coupled to primary P2, and the secondary is connected between the anode 33 and the grounded side of resistor section 34. A condenser 35 is connected in shunt with coil Si and tunes the latter to a frequency on one side of the operating intermediate frequency, whereas condenser 36 is in shunt with coil S2 and tunes the latter to a frequency located on the other side of the operating intermediate frequency.

The frequency control tube is shown by way of illustration as being of the pentode type, and its plate is connected by lead 31 to the high alternating potential side of the oscillator tank circuit. The cathode of control tube 38 is grounded through a grid bias network 39, and the control lgrid of tube 38 is connected to the anode side of resistor 34 through a path which includes the grid return resistor 40 and a lead 4|. The lead 4| is designated by the letters AFC and is to be understood as denoting the automatic frequency control bias path. The lead 4| is adapted `to be connected to ground by a switch 42, and it will be understood that the automatic frequency control bias path for tube 38 is operative `only when switch 42 is open. When the switch 42 is closed, then the AFC path is short circuited vto ground and is inoperative. The control grid Qf tube 38 is also connected to the upper end denser 43, a bypass condenser 44 being connected between the low alternating potential side of the tank circuit and ground.

In considering the operation of the AFC system of the receiver shown in Fig. 1 it is first pointed out that a high degree of selectivity in a superheterodynereceiver is of no great use unless it is possible to tune the receiver with a corresponding degree of accuracy, and thereafter to maintain this accuracy. As is well known, if the tuning of a superheterodyne receiver is inadequate the high selectivity may actually become a detriment. For example, in al1-wave, or multiband, receiver sets selectivity has been effectively increased 15 to 20 times over that usual for broadcast reception, merely because the received signal freqencies have been increased by that amount Without changing the intermediate frequency band width, i. e., at 20 megacycles (mc.) the nominal 10 kc. intermediate frequency band is only 0.05% of the base frequency. To meet this increase in selectivity in present-day receivers manual tuning means have been improvedby the employment of more smoothly vworking speedreducing mechanical movements to operate the variable tuning condenser.

Nevertheless the maintenance of proper tuning after the station signal has once been correctly brought in, is a problem that requires a reliable and accurate solution. Local oscillator drift, if not corrected by more or less frequent manual readjustment, is vcapable of mistuning the signal by many channels in the course of a few hours run. In the broadcast band, conditions are quite as serious if quality of reproduction is a consideration. The average broadcast set user does not tune the receiver well enough to obtain the best quality it is capable of giving; this is due not only to negligence, but to a great extent because of the lack'of necessary skill, and in the latter case the mechanical design of the set is a contributing factor. 'Ihese considerations show the need for supplementing the accuracy of manual or automatic tuning by an automatic fre quency control device. I

It will therefore be seen that it is the essentia object of the frequency discriminator and frequency control tube to cooperate to adjust the oscillator frequency so as to make it appear as if the signal carrier frequency has been adjusted precisely to the center of the receiver I. F. intermediate frequency band and to anchor it there in spite of small original maladjustments of tuning, or other causes that subsequently arise from thermal changes and the like. It will now be seen that the frequency discriminator network generates a bias, applied through lead 4|, which varies with changes of the predetermined I. F. value; the control tube 38 hasv its gain varied by the AFC bias, and the control tube functions to vary the local oscillator tank circuit in frequency. The

discrilninator and control units are so inter-related that if the I. F. carrier frequency tends to move away from the operating I. F. mid-band position, then the oscillator frequency changes sufficiently to restore the proper alignment. Such units, and their construction for performing these functions are generally disclosed in aforementioned U. S. Patent 2,294,100.

The two similar I. F. transformers, between i tube 2| and tube 3|, have their primaries P1 and P2 connected in parallel in the plate circuit .of the .driver tube 2|. `This composite primary is aange-45 .tuned nito .the tmideband ,frequency zof .the :.I. lband; in other -Words .the f composite vprimary lis tuned 'to- 460 kc. The secondaries S1 and .y Szfare .floosely coupled to .their :respectiveprimariesand are tuned to different freguencieslying :above zand below the zmid-band lfrequency by equal ramounts. Thus, thezsecondary'sSi maybe tuned 1:0462 rkc., and ithe lsecondarySz may .be Aftuned 'Lto-458 kc. The `'AVC diodeisrdriven 'from .the composite Iprimary .associated rwith the adriver ...tubea2.l, andtheadiode'loadZB not only'functions .-as^- such, but also' acts .tof damp v'the composite pri .fmary iny order to aid in de-couplin'g thesecondaries zfrom^ each other-.and lso. causes .them-.to act ..-like=,isolatedsingle tunedf circuits.

'The secondaries S1 andSz arezeach connect- .ediinto one of thegplates :ofltheradouble'diode .tube 31, and .the cathode'of-.thisitube is floatng-for direct :current voltage, One rof the diodesre- 1turns Ato'groundy and the -AFC bias output istaksen-'from the otherdiodemeturn. Theoutput .of fthendiscriminatorris the algebraic difference-between the rectified outputsof the t-Wo diodes. ;If .thef1I.iF. carrier 'frequency is- .off centerfrequency, 'tor 'the foperating .1. F. fvalue, 'towards `.the resonance of coil vS1, then the diode :including fancde hv32 willgproduce .thegreaterrectied voltage output of the two diodes, `and vitheigenerated AFC bias -will be negative `With .respect :to ground. .Conversely, if the I. F.-carrienfrequency:isyoi -in .the .otherdirectionmhe reverse will betrue. Thatzis to z-say, theAFC-bias lwill bepositivein zpolarity. When lthe fcarrier frequency is exactly :zalignedwith the ioperating .frequency of y 460 kc., fthe V:bias on .the grid of tube 138 "will equal rthe -..initial "Ibias A.value `thereof .sincef the AFC .bias will athen bezero.

Tt will be vnoted that the .AVC diode is Adriven from the driver tube'which .feeds the discriminatoranetwork. This modeof generating AVC bias 'is employed, .fin preference to fcleriving YAVC :inzput .energy fromthewcircuit1Il,because there is lthus obtained a higher Acarrier .input 1into .the l.AJ/'C rectifier. lA .considerable voltagefdelaymay be used cnethe AVC -.diode. .Another advantage, `.oftakingoif the AVC bias in this manneris that the signal modulation at the second'detector Lis '.not distorted bythe demodulation produced by .the AVC diode.

.If .the .AFC :driver `.tube Y.also rdrives the AVC ffrectier-its .output .-.is automatically .held near-ly constant no matter Where Vit -is connected. Therefore, the igrid of .thedriver `.tube ,may sbe .conne'cted .at the ,primar/y for secondary .of the ftransformer preceding the .last .1. lF. .amplifier ^tube. This has the `merit-'of fgiVi-nga `broader channel for vthe AFCand the :AVC than -for .the audio detector. The gain .of thelast -I. F. stage :should be considerably decreased to avoid-second ..detector overload.

As clearly expainedin .aforementioned U. S. :Patent 2,294,100, .maximum AFC sensitivity :is :obtained when .the di-scriminator characteristic .has the greatest slope. This .occurs `When the two secondaries .S1 .and Sz are so tuned that .points of maximum slope on their individual res- -`onance curvesare made lto coincide. The con- -trol circuitacts to vary the oscillatorifrequen'oy .and this variation may bethought of r.as if due 4rto :a Variation in capacityor .of aninductance.

The total tank circuit reactance maybe Lconsid- 'ered to consistzoftwo parts viz.,zthat.part ...physficallypresent as in v.the variable'tuning conden- .iser A.or '.'the .tuning coil, :an'dzthat part reflected #..byithetcontrclftubezeircuit. .'Ihelattersisrafunct-ion1;o`f'.the I.' F. .carrier-frequency, anditsmagni- .tude Varies withv the` carrier :frequency shift-from .1. midebandzfrequen'cy.

yIn aforementioned U. S. 'Patent 2,294,1001the .eontrolvtube has been shown as reecting capac- .fity into the tank circuit, or :the .plate resistance of thef`control'itube has beenconnected'in 'series with a reactive element in the tank circuit. vAd- 4fditional investigation and experimentation have revealed frequency [control circuits -Which lmake use-Tof a different principle. It .will be noted that the ,plate of-.control-tube `38 is coupled intofthe fhighpotential side ofthe tank circuit, and that .the ,grid of the control tube is excitedbyan alter- Q, .hating voltage which is .90 out of phase with .that 'appearing across the .tank circuit. The. plate ...current .in the control vtube is-then likewise out Moffpliase with the tank voltage,and accordingly the-,plate tocathode impedance of control tube o '38.looks like .a'reactance tothetank circuit. The sign of the -reactance ypresented to -.the :tank circuit .by .the .control tube depends vupon `.the nature of the impedance .across which :is edevelope'd .alternating voltage to .be fed to the input electrodes ofthe control'tube. The magnir` etude of the-.reflected ireactanceI depends .uponlthe value of the .-AFC .bias impressed upon .the control `,grid of -..control .tube 3.8,as this `variesthe mutual conductance, andlproportionately the ymagnitude -.of .the plate current, of vthe control .tub'e. 'In .:Fig. l1 lth-ere is'shown :one "method 'of .obtaining the out--of-,phase .excitation for lthe .grid vof the v'control tube,v and it Willfbe-'noted that this isac.- '.complished by taking the alternating voltage 5..-across-fresistance R. The effect of this arrange- .rmentfisfto produce -anfinductive effect across the A.tank colli-4. In' other Words,the plate to cathode Aimpedance of tube 'simulates an inductive freactancecacross coil I4. As thepplate currentof y.tube-:33 .vis I.decreased .the shunting effect .of the -refiected or .simulated `inductance :is decreased, and the Aeffective :inductance v:of the tank circuit v`Will therefore increase. 'I'his Hresults in a .dercrease-.of oscillator frequency.

.A .specific example will vbe .given to .illustrate .the operation of .the control tube '38. Assume that the -set is to beitunedzto. a signal .of 600. kc.; .thelocal oscillator'frequencydf higher Willhave to l.be y1.060 kc :to :.produ'cexan 1I. F. of `460 kc. the.tuner'device .fis adjusted from 500 rto '600:kc., 'the .oscillator condenser Al'3 'tunesth'ertank circuit from :960 .to l060.kc. When .the tank circuit lis within2rkc., .for example, of 2106.0 kc. (i058 kc.)., the .I.:F. yvalue :is 458 kc. :Since this is the 'frequency of the circuitcsz, rectified .voltageWill be developed across resistor 534". lThefcontrol .gr-1d of l"tube .138 fwill, therefore, become .less negative than :its initial :negative bias value with respect 'toground due to its connectionthrough lea'd 1M .-.andresstor'llftothe cathode side of resistor-'34'. 'The ..piate current A"flow of -tube 318 increases; this 'will :result .in ian .increase in .the inductive fre- :actance.refiexedfa'cross coil l4. Thua'tlie effective :indue-tance in "the tank 'circuit decreases; the -65 .oscillator frequency, therefore, increases.

The 4,increase vin f frequency f of the tank 'circuit Will besuch as to bring the local oscillation fre- Lquency to 1060.1kc. Thenfthe proper'I. F. value-df `146i) kc. iszsecuredgandfthe Iset soundstunedin" l-to'th'eilistener. .Thehreverseaction takes place, cof icourse, .when fthe Aset `is tuned away ffrom Athe :idesii'ed-.settingof 600.kc. vWithin Z'fkc. onI either :sidey UfGOOkdthe'AFC system will act to' fanchor" fthe tuninfgfofthe receiver. -When the-receiveriis 7B Jtuned :beyoncl the .permissible 'limits of a carrier setting, the AFC action is suspended until the limit of the adjacent channel is reached. In this way the nonmally highly selective superheterodyne receiver is rendered easily tunable; a range of some 4 kc., for example, is provided Within which the set may be tuned and yet distortionless reproduction be secured. The same operation is secured when the local oscillator circuit constants vary due to thermal, or other, effects. The AFC acts to bring the oscillator frequency back to the desired operatingr value so as to produce the operating I. F.

There are numerous methods of obtaining the out-of-phase grid voltage for the control tube 38. In order to make it clear to those skilled in f the art how such different modes of procedure may be achieved, the following analysis of the operation of tube 38 and its circuits is furnished. It is to be clearly understood, however, that the analysis is theoretical in nature, and in no way aiects the demonstrated operation of the reactance control network.

It can be shown, in general, for an oscillator network of the type illustrated in Fig. 1 (that is, the tank circuit and the tube 38 with its circuits) that the combination of the control tube 33 and the oscillator presents to the tank circuit an eiective impedance which is the negative reciprocal of the control tube impedance with respect to the resistance product of both oscillator and control tubes. For example, a capacity is the reciprocal of an inductance with respect to a resistance product; the reciprocal of a shunt combination is a series combination.

A practical Iresistance value for resistor R in Fig. 1, for a broadcast local oscillator, is about 100 ohms. With the Gm mutual conductance of tube 38 equal to 1000 micromhos, the frequency change is about percent. This permits a variation of plus or minus 25 kc. at 1 mc., which seems ample for the purpose. The tuned impedance of the tank circuit is unaffected by the control action. Constant oscillator amplitude with irequency change is one of the advantages of the present arrangement.

The series resistor R is not coni-ined to the inductive leg of the tank circuit. For example, and as shown in Fig. 2, the resistor R1 is placed in the capacitive leg of circuit I3-I4. In this case the control tube cathode to plate impedance will appear as a resistance-capacity arm across the tank circuit; increase in the Gm of the control tube will now lower the oscillator frequency. I-t is necessary, in this case, to reverse the AFC connection to the disfcriminator network. Lead 4I is, therefore, connected to the anode side of resistor 34 and the anode side of resistor 34 is grounded.

Another method of securing the outoiphase grid voltage for tube 38 is shown in Fig. 3, Here the reactive element is condenser C which is connected in series with the oscillator tickler coil I4. Condenser C is grounded, and choke 50 is used to feed the direct current voltage to the plate of oscillator tube I2. The current owing in the external plate circuit of tube I2 is in phase with the tank circuit voltage. A reactive element in the plate circuit will, therefore, develop an out-of-phase voltage which is impressed upon the control grid of tube 38. The cathode to plate impedance of tube 38 presents a negative inductance across the tank circuit; the net result is an increase in eiective inductance in the oscillator tank circuit. The AFC connection must,

then, be reversed in polarity with respect to that shown in Fig. 1.

If the capacitive reactance C is replaced by an inductance coil then a negative capacity will be reflected across the tank circuit. If a negative inductance, say the mutual inductance of a transformer, weraused in place of C, it would be reflected as a positive capacity. Any resistive element reflects asa negative resistance, or conductance, and tends to aid the feedback of the oscillator.

In Fig. 4 is shown a modication of the method of securing out-of-phase voltage; it differs from that of Fig. 3 in that the out-of-phase voltage is taken from across the tickler coil I4'. The plate to cathode impedance of the control tube, in this case, presents a negative capacity across the tank circuit. It is not necessary to change the AFC connection to the discriminator network from that shown in Fig. 1.

It may be that the reflected reactance from the tank circuit will give rise to an ln-phase component across coil I4. This may be eliminated by using the arrangements shown in Figs. 5 and 6. In Fig. 5 the coil L1 is coupled to tickler I4', but it is not coupled to tank coil I4. The out-ofphase voltage across coil L1 is impressed upon the control grid of tube 38 through coil Lz; the latter coil is coupled to ycoil L1. The plate to cathode impedance at control tube 38 presents a negative capacity across the tank circuit I3, I4; the AFC connection 4I is the same as that in Fig. 4. In Fig. 6 there is shown the effect of reversing the end connections of coil Lz. The plate to cathode impedance of tube 38 now presents a positive capacity across the tank circuit. The AFC connection 4I must be reversed in polarity as shown. It is, further, pointed out that the circuit of Fig. 3, wherein the out-of-phase voltage is derived from across the condenser C, can be used to avoid the in-phase component of Fig. 4.

While there have been indicated and described several systems for carrying this invention into effect, it will be apparent to one skilled in the art that this invention is by no means limited to the particular organizations shown and described, but that many modications may be made without departing from the scope of this invention.

What is claimed is:

In combination, a tuned tank circuit including a coil and a condenser connected in parallel, an electron discharge device regeneratively connected to the tank circuit whereby oscillations are set up at a frequency determined by the tuning of said tank circuit, a reactance tube having an anode, a cathode, and a grid, the anode and cathode being connected to the tuned circuit so as to vary the effective tuning of the tank circuit in accordance with electron current iiow from the cathode to the anode of said reactance tube, a coil connected between the grid and cathode of said reactance tube, a pair of serially connected lcoils for subjecting the grid and cathode of said reactance tube to oscillatory voltages in quadrature with the oscillatory volt-v ages across said tank circuit, one of said serially connected coils being inductively coupled to the coil of said tank circuit and the other of said serially connected coils being inductively coupled to the coil connected between the grid and cathode of said reactance tube, and said regenerative connection comprising said serially connected coils, and a circuit connected to electrodes of said reactance tube for varying the 11 I2 electron 'ow from they cathode; tothe anode UNITEDSTATES' PATENTS thereof, and thereby the frequency of oscillations set upv by saidV device and, banli crcuiigirr accordlugrl Roume MayD'telgzg ance withy the control potentlals. 5 1,788,533 Marrison Jan. 13 1931 Aug. Administratriof the Estate of Charles Travis, 2,027,291* Roblsm Jan- 7 1936 Deceased, 2,294,100 Travis Aug. 25, 1942 2,357,984 Travis Sept. 12, 1944 REFERENCES CITED lo 2,374,265 Baker et al. Apr. 24, 1945 The following references are of record: the OTHER REFERENCES me' of this patent: Electronics, page 18, January 1935. 

